Mask for etching enlargement

ABSTRACT

A mask has a metallic material, preferably cold-rolled steel, covered by an etchant-resist layer on both sides. On one side the layer has a circular area of diameter X2 void of etchant resist, and on the second side the layer has a circular area of diameter X3 and an annular area of outer diameter X1 both void of etchant resist. The three areas are coaxially aligned.

United States Patent [50] Field ofSearch 355/26, 77,

[72] lnventor John J. Frantzen North Saint Paul, Minn. 125 [21 l 2 1969Primary Examiner-Samuel S. Matthews [22] 1 e I AssistantExaminer-Michael D. Harris [45] Patented Sept 1971 Attorney-Stryker andJacobson [73] Assignee Buckbee-Mears Company a St. Paul, Minn.

ABSTRACT: A mask has a metallic material, preferably coldrolled steel,covered by an etchant-resist layer on both sides. [54] MASK FOR Q FENLARGEMENT On one side the layer has a circular area of diameter X,void of 4 Claims 7 Dram"! etchant resist, and on the second side thelayer has a circular [52] US. Cl 355/26, area of diameter X and anannular area of outer diameter X 355/77, 355/ 125 both void of etchantresist. The three areas are coaxially [5 1] Int. Cl 60;"; 21132 aligned.

1. flz y I 4 *h X 2 F /v i m I T i 35 30 PATENTEBsEmmn 3.609033 sum 1 or2 INVENTOR JOHN J. FRANTZE/V ATTORNEYS l. Fieldof the Invention Thisinvention relates generally to making shadow masks for coloredtelevision tubes and, more specifically, for photopn'nting unusualpatterns in register that are suitable for etching enlargement in atwo-step etching process.

2. Description of the Prior Art In a colored television picture tube,the shadow mask or aperture mask is located between the electron guns attherear of the tube and the phosphor-coated faceplate at the front'orviewing face of the tube. Electron beams pass through the openings orapertures inthe shadow mask and impinge upon a suitable color producingphosphorus dot on the faceplate. Behind each of the openings of theshadow masks are three phosphorus dots, a triad, one dot for each of thethree primary colors. During the asse'mbly of the picture tube theshadowmask is used as a mask or master for forming the phosphorus dots on thefaceplate of the television tube. This is usually done usingphotographic techniques which are well known in the industry today.

In order to produce the maximum possible brilliance at the face of thetube when the phosphor is struck by the electron beam but at the sametime to focus the beam on the proper triad, it is necessary to have thephosphor dots smaller in diameter than the diameter of the apertures ofthe shadow inask. To facilitatethe screening of the phosphorus dots withthe existing apertures of the shadow mask, it requires a temporarystepping down or reduction in the diameter of the apertures by partiallyfilling the apertures with a second material tha'tis different from themask material. The, after using the mask for forming the pattern for thephosphorus dots on the faceplate of the tube, the filling material canbe removed to enlarge the holes in the aperture mask so thatthe maximumbrilliance can be obtained in the television tube.

' Another method of obtaining smaller holes in the shadow mask for thedeposition of the phosphorus dots is to etch a set ar first circularopening in the shadow mask which have a diameter suitable for use as apattern for forming the phosphorus dots on thefaceplate of thetelevision tube. Then after layingthe phosphorus dot pattern on thefaceplate of the television tube the holes are subjected to etchingenlargement by controllably spraying etchant on the mask.

The present invention comprises the method of printing and compensationfor forming a suitable pattern in register in photo resist for producingthe etched configuration of a set of "circular openings with acorresponding set of undercut annular areas located around the circularopenings. During the two-step etching process a set of larger diameterannular areas which are coaxially located with respect to the first setof circula'r'openings are etched partially through the *mask. Inal'a'ter etching step the annular area'which was etchedpartially throughthe first etching step is etched completely through,

thus causingthe material inside the annular area to'drop out leaving aset of largerdiameter openings in the shadow mask.

As the etching pattern does not exactly follow the resist pattern, andas the relative dimensions between the circular openings and the annularareas are quitesmall, it is difficult to uniformly etch awaythe'material around the circular openings to cleanly remove the annularring surrounding the central opening.

SUMMARY Briefly, the present'invention comprises a process where theholes in a shadow mask are first etched to produce an opening for layinga phosphorus dot pattern and then etched to remove an annular ring thusleaving the opening enlarged to the set of pinholes and onto a secondphotosensitive plate and then developing the negative formed on thesecond photosensitive plate. Next, the negative master plates are placedin optical registration and put into a suitable printing apparatus toprint the etching pattern in photoresist on opposite-sides of the mask.In the next step the mask is immersed in an etchant until a first set ofcentral apertures are etched out. next, the mask'is used to lay thephosphorus dot pattern and finally, the mask is immersed in etchantuntil the material surrounding the aper ture has been removed to providethe enlarged aperture opening of the desired size.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows material used for makingthe negative pattern on a plate glass;

FIG. 2 shows the frontside patterned image on a suitable glass plate;

FIG. 3 shows the camera setup and the pinhole negative for use with theimage that is projected on one side of a master pattern used for makinga shadow mask;

FIG. 4 shows the backside patterned image whichis formed on the oppositeside of the shadow mask;

FIG. 5 shows a cross-sectional oblique view of the etchantresist patternlocated on opposite sides of the material;

FIG. 6 shows the cross-sectional appearance of the mask after etchingpartially through the mask; and

FIG. 7 shows the cross-sectional appearance of an aperture as thesurrounding annulus is etched free from the mask.

DESCRIPTION OF THE PREFERRED METHOD FIG. 1 showsa portion of a suitableopaquematerial for use in making an enlarged pattern for use in thephotoprinting process. An example of a suitable opaque material is anopaque material that comes in a roll and is marketed under'the tradename Studnite. Preferably, the operator cuts thepattern at approximatelytimes scale to minimize and reduce errors in forming the pattern. Aftercutting the;pattem outline in the opaque material, the areas 1.1, 12,and 13 (H6. -l,-).are

carefully stripped away from the backing material. 'Note,the

pattern also includes guidelines 11 located horizontally and verticallyto enable the operator to properly align'the pattern with respect to thecamera. Next, the materialis contacted'to a first glass plate and thenrecontacted to a second glass plate 16 (FIG. 2) to put the pattern innegative contrast. The negathe contrast patterns are identified by 110,12a, and 13a,

Next, the single pattern in negative contrast onglass plate '16 -'isplaced in a copyboard holder 17. Similarly, a backside'pattern 27 (FIG.4) is formed on a glass plate 27.

After placing pattern plate 16 in copyboard holder 17, a suitable sourceof illumination such as a set of mercury vapor lights 18 are turned onbehind the copyboard. At thisti'me negative 8 and pinhole board 9 arenot in the camera as.pat-

tern plate 16 has to be aligned prior to exposing the negatives.

respect to the lineup pattern. Typically, the lineup pattern is also aglass plate that has a set of mutually perpendicular lines located inthe center of the glass plate. With the lights on behind the copyboard,the image on the copyboard pattern plate is lined up with respect to thecenter of the lineup pattern in the back of the camera. After obtainingproper alignment of pattern plate 16, the operator fastens pattern plate16 to prevent movement during the subsequent steps and removes thelineup pattern plate from the back of the camera. In the next step, thelights are turned off and pinhold board 9 is placed into the properposition in the back of the camera (FIG. 3). Next, the operator places aphotosensitive plate 8 behind the pinho ld board. Photosensitive plate 8is positioned so that it lies in a plane parallel to the pinhole board.As the lens has been removed from the camera 10 an image can beprojected from the copyboard through each of the pinholes and ontophotosensitive plate 8 when light 18 is turned on.

Turning light 18 on produces a set of images on photosensitive plate 8which are identical in appearance to the copyboard image or patternplate 16, because they all have been formed from the same copyboardimage on pattern plate 16.

In order to obtain the backside pattern for the shadow mask, the sameprocedure of placing the pattern plate 27 in copyboard holder 17 andaligning pattern plate 27 with respect to a lineup pattern is repeated.Care must be taken to ensure that the spacing of the pattern plates fromthe pinhole board and the spacing of photosensitive plate from thepinhole board is correct to ensure that the proper reduction in imagesize is obtained.

In utilizing this process to form a multiple negative aperture patternfrom a master frontside pattern and a multiple negative aperture patternfrom master backside pattern, it is necessary to utilize either the samepinhole board or an identical pinhole board to obtain multiple negativefrontside patterns and multiple negative backside patterns that can beplaced in alignment. By utilizing the same pinhole board, it eliminatesthe possibility of misalignment of registration due to irregularitiesfrom pinhole board to pinhole board.

Once the photosensitive plates have been exposed by light projectablethrough the image on the copyboard and the openings in the pinholeboard, the photosensitive plates can be developed. These negativescontaining a multiple frontside and backside patterns can be used as amaster pattern to mask off the photoresist material on both sides of themask. However, if the negative master patterns are printed inmisalignment, the two-step etching process cannot be satisfactorilyaccomplished. For example, if there is misalignment so that the circularbackside pattern is not coaxial with the annular frontside pattern, itwill produce regions where the etchant cannot etch completely throughthe mask without enlarging the central opening beyond the preferredsize. Obviously, such masks are unsuitable for use in colored televisionindustry as the electron beams cannot be focused properly through theapertures unless the apertures are properly sized.

The foregoing has been a description of the process of forming thefrontside and backside pattern on suitable negative master patterns. Inaddition to forming and then printing the patterns in registration onthe frontside and backside of the mask, it is necessary to carefullyselect the relative pattern dimensions to enable the operator to cleanlyremove the excess material with the two-step etching process. The aspectof the dimensional compensation for obtaining a pattern having asuitable undercut surrounding region will now be described.

In order to appreciate the difficulties in forming a suitable patternthat can be used in a two-step etching process, an ex ample of thetypical dimensions involved in the apertures of the shadow mask aregiven hereafter, although no limitation is intended thereto. In atypical shadow mask the diameter of the central opening a (FIG. 6) isapproximately 0.008 inches. After the shadow mask has been used as apattern to lay the phosphorus dots on the faceplate, it is necessary toenlarge the central opening b (FIG. 7) to approximately 0.015 inches.FIG. 5 shows a material 30 having a layer of patterned etchant resist 31on one side and layers of patterned etchant resist 32 and 33 located onthe opposite side. Resist layer 31 has a central area 34 which is voidof etchant resist and similarly located on the opposite side are twoareas 35 and 36 which are also void of etchant resist. Referencecharacter X designates the diameter of circular area 34 and referencecharacters X and X designate the respective outermost dimension of area25 and area 36. Reference character L designates the width of area 25while reference character T designates the thickness of material 30.

FIG. 6 shows a greatly enlarged aperture '38 having a central opening ofdiameter a in cross section after the first etching step. Note, theoutline of the etched material 30 with the relatively bowl-shaped sides20 and the smaller angular areas 21 and 22. Also, note how the etchanthas undercut the corners of the resist.

Briefly, in order to obtain a mask having a suitable undercutconfiguration such as shown in FIG. 6, it is necessary to print thepattern in photoresist so that the outermost dimension Xhd l on one sideof the mask is approximately equal to the outermost dimension X on theopposite side of the shadow mask (see FIG. 5).

Also, in order to properly undercut the mask, it is necessary to limitthe width of the annular area L to slightly less than one-fourth thethickness T of the shadow mask. Having the dimension L too large causesthe material in area 35 to etch completely through to sidewalls 20before the desired central opening 38 is obtained. Having the width Ltoo small prevents the etchant from etching to the proper depth so thatringlike member 40 (FIG. 7) can be removed in a second etching stepwhile still maintaining the integrity of the final dimension b.

Located in photoresist within the central area having a dimension X is asecond central area having a dimension designated by X This diameter issubstantially equal to the diameter of the central opening a through themask (see FIG. 6). During etching the etchant undercuts the resist andgenerally angles inward to produce the desired central opening ofdiameter 0 shown in FIG. 6.

In order to etch a typical cold roll steel shadow mask, the operatorlowers the shadow mask containing the photoresist pattern, which isshown in FIG. 5, into a solution of ferric chloride etchant. The shadowmask remains in the etchant until the etchant etches away the centralcircular areas thus leaving an opening of diameter a. This can bedetennined by visual inspection and by monitoring the time the mask isin the etchant. When the mask is immersed in etchant, the largerdiameter open area 35 etches faster into the shadow mask than thecorresponding smaller areas 35 and 36 which are located on the oppositeside of the mask. After the apertures 38 appear uniform throughout themask, the mask is removed from the etchant and washed to prevent anyetchant clinging to the mask from enlarging or further undercutting themask. In a typical mask areas 35 and 36 etch approximately onethird ofthe thickness of the mask while on the opposite side of the mask thecenter of the larger area 34 etches to a depth of approximatelytwo-thirds the thickness of the mask in the same amount of time. Bylimiting the width of the annular area L so that it is approximatelyonefourth of the thickness of the mask and utilizing the bowl-shapedetching pattern, one can produce an area surrounding the opening ofthickness c which can be etched away in a later etching step. After themask has been used to lay the phosphorus dot pattern, the mask isimmersed in etchant until the area of thickness c has been etchedthrough (FIG. 7). This causes ringlike member 40 to drop out leaving anoperator having an enlarged final diameter b. The resist can now beremoved and the mask placed in the television tube.

Although the process has been described in terms of circular openingsand circular secondary areas, it is apparent that the process could bealso used to print oblong, hexagonal or other shaped openings.

lclaim:

1. An aperture mask suitable for etching enlarging comprising: ametallic material having a thickness T; a first layer of etchant resistlocated one side of said material and a second layer of etchant resistlocated on the opposite side of said material; said first layer ofetchant resist having a first circular area of diameter X beingsubstantially void of etchant resist; said second layer of etchantresist having a second circular area having a diameter X beingsubstantially void of etchant resist, said second layer of etchantresist having an annular area substantially void of etchant resist, saidannular area having an outer diameter X said outer diameter X beingsubstantially equal to the diameter X, of said first circular area.

2. The inventions as described in claim 1 wherein said first circulararea, said second circular area and said annular area are located incoaxial alignment. 1

3. The inventions as described in claim 2 wherein the width

1. An aperture mask suitable for etching enlarging comprising: ametallic material having a thickness T; a first layer of etchant resistlocated one side of said material and a second layer of etchant resistlocated on the opposite side of said material; said first layer ofetchant resist having a first circular area of diameter X2 beingsubstantially void of etchant resist; said second layer of etchantresist having a second circular area having a diameter X3 beingsubstantially void of etchant resist, said second layer of etchantresist having an annular area substantially void of etchant resist, saidannular area having an outer diameter X1, said outer diameter X1 beingsubstantially equal to the diameter X2 of said first circular area. 2.The inventions as described in claim 1 wherein said first circular area,said second circular area and said annular area are located in coaxialalignment.
 3. The inventions as described in claim 2 wherein the widthof the annular area L is approximately one-fourth the thickness T ofsaid material.
 4. Inventions as described in claim 3 wherein saidmaterial compromises cold-rolled steel.